APPARATUS FOR REPRODUCING INFORMATION

Abstract: This apparatus reproduces data from a recording medium by a partial response method and decodes data by use of a shift register by the Viterbi decoding technique. The shift register has flip-flops Da j which latch data and are connected in series, and the XOR gates (31a to 31d) connected between the flip-flops Da j and used as operating means for calculating the exclusive disjunction of the outputs of the flip-flops so that CRC operation is executed.

Description: TECHNICAL FIELD The present invention relates to an information reproducing apparatus suitable for use in a case where information recorded on, for example, a magnetic disc, a magnetic tape, an optical disc, a magneto-optical disc and so on is decoded with a maximum likelihood decoding method such as a Viterbi decoding method or the like. BACKGROUND ART A partial response is used as a modulation code used in a magnetic recording and reproducing apparatus or an optical recording and reproducing apparatus. Of kinds of the partial response, PRS (1,1) (class I), PRS (1,-1), PRS (1,0 -1) (class IV) and so on are used frequently. An arithmetic circuit 101 shown in FIG. 6(a) uses the PRS (1,0,-1). Arithmetic circuits 102, 103 shown in FIG. 6(b) use the PRS (1,-1). A system polynomial G(D) of the PRS (1,0,-1) is G(D)=1-D 2 and a system polynomial G(D) of the PRS (1,-1) is G(D)=1+D, where reference letter D represents a delay operator. The arithmetic circuit 101 is a circuit which successively outputs data of 1, 0 and -1 when an independent logic 1 is input. The arithmetic circuits 102, 103 are circuits which successively output data of 1 and -1 when an independent logic 1 is input. The arithmetic circuit 101 (PRS(1,0,-1)) shown in FIG. 6(a) has the system polynomial of G(D)=1-D 2 and hence always calculates input data y k inputted at a certain sampling time k as a two previous sample y k-2 . Accordingly, an odd number sample and an even number sample are substantially independent of each other and can be regarded as independent series of the partial response PRS (1,-1). The circuit shown in FIG. 6(a) is equal to a circuit shown in FIG. 6(b) in which the odd and even number samples of input data are respectively supplied to and processed by the arithmetic circuits 102, 103 for calculating the partial response PRS (1,-1) by switching a switch 104 and outputs of the arithmetic circuits are synthesized by a switch 105 to be output. Decoding in which the arithmetic circuits 102, 103 (PRS (1,-1)) are used with being interleaved is substantially the same as decoding carried out by the arithmetic circuit 101 (PRS(1,0,-1)). Here, the partial response PRS (1,0,-1) will be explained by way of example. The partial response PRS (1,0,-1) itself has property of permitting an error to be propagated and hence is followed by a risk such that if one bit error is caused under a certain condition, then a fatal error is caused. Therefore, in order to prevent such error from being caused, it is necessary to precode data before recording them. This pre-coding can be carried out by effecting a reverse conversion of the partial response. FIG. 7 shows a total arrangement of a system for modulating and demodulating the partial response through such precoding. In FIG. 7, a pre-coder 111 carries out a process of 1/(1-D 2 ). Recorded data are converted by the pre-coder 111 into pre-coded data varying between a value 1 of the recorded data and a value -1 thereof by utilizing a correlation between data of the recorded data and then output to a recording channel circuit 112. The recording channel circuit 112 is not an especially provided circuit but a function which is inherently possessed by a magnetic recording and reproducing system and is represented as an equivalent circuit. In this circuit (that is, when the data are magnetically recorded and then reproduced), an arithmetic processing circuit 113 subject the output from the pre-coder 111 to an arithmetic processing of (1-D). At this time, a noise generated in a practical magnetic recording channel is treated as one to be added to a result of the calculation by an adder 114. Data to which the noise is added (data reproduced after magnetically recorded) are output to an arithmetic processing circuit 115 at the succeeding stage of the recording channel circuit 112. The arithmetic processing circuit 115 subjects the output from the recording channel circuit 112 to an arithmetic processing of (1+D). As shown in FIG. 8, a signal output from the recording channel circuit 112 is set at any one of three levels of {-2,0,+2} if the signal level ranges between values ±2. When the signal is decoded by a decoder 116 so as to be an original binary data (1 or 0), it can be considered to take a three-level detecting method using fixed threshold values or a Viterbi decoding which is a maximum likelihood decoding method. The three-level detecting method is a method in which two threshold levels having predetermined fixed values are respectively set between values 0 and +2 and between values 0 and -2 and data are decoded by determining whether a signal level sampled at a sample point is larger or smaller than the threshold levels. The method has an advantage such that the circuit can be very simplified but has a disadvantage such that a detection ability is comparatively low. On the other hand, the maximum likelihood decoding method (the Viterbi decoding) is a method in which data are decoded by using values at previous and subsequent sample points to detect a series (path) of the data obtained after decoded and a maximum likelihood series (path) is estimated on the basis thereof. As compared with the three-level detecting method, the maximum likelihood decoding method has high detection ability and can reduce a bit error rate to an extent from two figures to one figure when the same data are decoded by both of the methods. Next, there will be described an example of a circuit used when the decoder 116 is formed of a Viterbi decoder. Before the circuit is explained, the Viterbi decoding will be explained in order to understand the circuit. Having the system polynomial of 1-D 2 , a system using the PRS (1,0,-1) has four states. If data are derived from the system at every other bit, then such derived data form one system (that is, PRS(1,-1)), which has two states because its system polynomial is 1-D. A state transition diagram of the PRS (1,-1) is shown in FIG. 9. If a value 1 is input when the state of the PRS (1,-1) is a k-2 = -1, then the state thereof is changed to a k = +1 and a value 2 is output. If a value -1 is input, then the state thereof is changed to the same state as an original state thereof, i.e., a state of a k = +1 and a value 0 is output. Further, if a value 1 is input when the state thereof is a k-2 = +1, then the state thereof is changed to a k =-1 and a value -2 is output. If a value -1 is input, then the state thereof is changed to the same state as an original state thereof, i.e., a state of a k = -1 and a value 0 is output. A trellis diagram (likelihood seeking diagram) (hereinafter referred to as a trellis) corresponding to the state transition diagram shown in FIG. 9 is shown in FIG. 10. In the trellis, if a sample value (which is the output from the arithmetic processing circuit 115 in this case) y k is input at a certain sampling time k , a branch metric (which corresponds to an instantaneous standard of a likelihood) from the state of a k -2 to a state of a k is indicated in the form of a value obtained by multiplying a squared error of the sample value y k by -1 (-(y 2 -0) 2 , -(y 2 -2) 2 , -(y 2 +2) 2 ,···). The Viterbi decoding finds a path through which a sum of all the branch metrics becomes maximum. Respective path metrics (which correspond to integrals of likelihood path) L k + and L k - up to a certain sampling time k under the states of a k = +1 and a k = -1 can be represented by using values L k-2 of the path metrics up to one previous sampling time k -2 as shown in the following equations (1) and (2). L k + = max {L k-2 + + [-(y k -0) 2 ], L k-2 - + [-(y k -2) 2 ]} L k - = max {L k-2 + + [-(y k +2) 2 ], L k-2 - + [-(y k -0) 2 ]} Here, max {A,B} represents selection of larger number between the values A and B. In order to detect a most proper path while computing the metric, three squaring circuits, six adders and two comparators are generally required. Then, instead of calculating every path metric accurately, an algorithm in which a differential metric is used can be used in order to simplify the circuit. A Viterbi algorithm used when there are only two states will be considered. The Viterbi algorithm is operated such that it determines data while selecting one path in which likelihood in reaching each state at a certain time k becomes largest. The above-mentioned decoding circuit (decoder 116) is used to realize the Viterbi algorithm accurately. Difference between path metrics (a differential metric) in the states of a k = +1, -1 can be expressed by the following equation: ▵Lk = Lk + - L k - In case of L k-2 + + [-(y k -0) 2 ] > L k-2 - + [-(y k -2) 2 ] (the case of large likelihood in transition from a state of a k-2 =+1 to a state of a k =+1), the path metric L k + is determined from equation (1) as L k-2 + +(-(y k -0) 2 ]. In case of L k-2 + + [-(y k -0) 2 ] ≤ L k-2 - + [-(y k -2) 2 ] (the case of large likelihood in transition from a state of a k-2 =-1 to the state of a k =+1), the path metric L k + is determined therefrom as L k-2 - +[-(y k -2) 2 ]. On the other hand, in case of L k-2 + + [-(y k +2) 2 ] > L k-2 - + [-(y k -0) 2 ] (the case of large likelihood in transition from the state of a k-2 =+1 to a state of a k =-1), the path metric L k - is determined from equation (2) as L k-2 + +[-(y k +2) 2 ]. In case of L k-2 + + [-(y k +2) 2 ] ≤ L k-2 - + [-(y k -0) 2 ] (the case of large likelihood in transition from the state of a k-2 =-1 to the state of a k =-1), the path metric L k - is determined therefrom as L k-2 - +[-(y k -2) 2 ]. To summarize them, the path metric L k + has two different values obtained in case of 4>4y k -▵L k-2 (case of large likelihood in transition from the state of a k-2 =+1 to the state of a k =+1) and in case of 4≤4y k -▵L k-2 (case of large likelihood in transition from the state of a k-2 =-1 to the state of a k =+1). The path metric L k - has two different values obtained in case of -4>4y k -▵L k-2 (case of large likelihood in transition from the state of a k-2= +1 to the state of a k =-1) and in case of -4≤4y k -▵L k-2 (case of large likelihood in transition from the state of a k-2 =-1 to the state of a k =-1). Accordingly, the differential metric ΔL k expressed in equation (3) is varied depending upon each of four cases (=2 x 2), i.e., cases of (C + 1) and (C-1), (C + 2) and (C-2), (C + 1) and (C-2), and (C + 2) and (C-1). In case of 4>4y k -▵L k-2 and -4>4y k -▵L k-2 (case of a survivor path becoming a pattern of a state <+1> → the state <+1> and of the state <+1> → a state <-1>), i.e., of -4>4y k -▵L k-2 , the differential metric ΔL k is ▵L k = {L k-2 + + [-(y k -0) 2 } - {L k-2 + + [-(y k +2) 2 ]} = L k-2 + - y k 2 - L k-2 - + y k 2 + 4y k + 4 = 4y k + 4. Further, in case of 4≤4y k -▵L k-2 and -4≤4y k -▵L k-2 (case of a survivor path becoming a pattern of the state <-1> → the state <-1> and of the state <-1> → the state <+1>), i.e., of 4≤4y k -▵L k-2 , the differential metric ▵L k is ▵L k = {L k-2 - + [-(y k -2) 2 } - {L k-2 - + [-(y k +0) 2 ]} = L k-2 - - y k 2 + 4y k - 4 - L k-2 - + y k 2 = 4y k - 4. In case of 4>4y k -▵L k-2 and -4≤4y k -▵L k-2 (case of a survivor path becoming a pattern of the state <-1> → the state <-1> and the state <+1> → the state <+1>), i.e., of -4≤4y k -▵L k- 2 <4, the differential metric ΔL k is ▵L k = {L k-2 + + [-(y k -0) 2 } - {L k-2 - + [-(y k -0) 2 ]} = L k-2 + - y k 2 - L k-2 - + y k 2 = ▵L k-2 . In case of 4≤4y k -▵L k-2 and -4>4y k -▵L k-2 (case of a survivor path becoming a pattern of the state <-1> → the state <-1> and of the state <+1> → the state <+1>), summarization of these equations indicates 4≤4y k -▵L k-2 <-4. Therefore, the above case cannot happen. Judging from the above description, equation (3) can give a value of ΔL k depending upon size of the term 4y k -▵L k-2 and the value is given as shown in the following equation (4). Accordingly, in case of only two states (of a k = +1 and a k = -1), only the following three kinds of survivor path patterns are possible; the state <-1> → the state <-1> and the state <-1> → the state <+1>; the state <-1> → the state <-1> and the state <+1> → the state <+1>; and the state <+1> → the state <+1> and the state <+1> → the state <-1>. Here, the three kinds of possible survivor path patterns are represented by three kinds of two-letter symbols, i.e., →↑, →→ and →↓. Since inequalities used for discrimination of cases in equation (4) include a term of 4y k -▵L k-2 as a common comparative element, the value of the term is compared with a value 4 or -4 to determine size of the value. Therefore, it can be determined which of the above survivor path patterns the survivor path pattern is. In other words, calculation of the differential metric permits a survivor path, which survives in a process of the calculation, to be determined without calculating the path metric itself, and data can be decoded on the basis of the obtained survivor path. When the value of the differential path metric is transformed by change of variables to ΔL k = 4y p - 4β with setting a value y p as a sample value at a point (location P) where any one of paths except a parallel path (→→), i.e., an upward divergence (→↑) or a downward divergence (→↓) occurs in the trellis and with setting a value β as a so-called correction term, equation (4) can be expressed as shown in the following equation (5). When left and right sides of equation (5) are compared, if an upper or lower equality therein is established, i.e., the upward divergence (→↑) or the downward divergence (→↓) of the survivor path pattern occurs, it is understood that the value β has a value 1 or -1. Accordingly, the value β represents a divergence direction at the point (location p) where the first upward divergence (→↑) or the first downward divergence (→↓) caused before the present point occurs (whether the survivor path pattern at the point (location p) is one of the upward divergence (→↑) or the downward divergence (→↓). If the first divergence caused before the present point is the upward divergence (→↑), for example, i.e., if β =+1 is established, then the survivor path pattern at the present point is determined as the upward divergence (→↑) in case of 0 ≤ y k -y p , the parallel path (→→) in case of -2 ≤ y k -y p < 0, and the downward divergence (→↓) in case of y k -y p < -2 by substituting a value 1 for the value β in the inequality of equation (5) used for discrimination of cases (FIG. 11). Further, in this case, when the left and right sides of equation (5) are compared, the values β and y p are updated in a manner of y p ← y k and β ← +1 in case of 0 ≤ y k - y p , y p ← y p and β ← β in case of -2 ≤ y k - y p < 0, and y p ← y k and β ← -1 in case of y k - y p < -2 (FIG. 11). Similarly, if the first divergence caused before the present point is the downward divergence (→↓), for example, i.e., if β = -1 is established, then the survivor path pattern at the present point is determined as the upward divergence (→↑) in case of 2 ≤ y k -y p , the parallel path (→→) in case of 0 ≤ y k -y p < 2, and the downward divergence (→↓) in case of y k -y p < 0 by substituting a value 1 for the value β in the inequality of equation (5) used for discrimination of cases. When the left and right sides of equation (5) are compared, the values β and y p are updated in a manner of y p ← y k and β ← +1 in case of 2 ≤ y k - y p , y p ← y p and β ← β in case of 0 ≤ y k - y p < 2, and y p ← y k and β ← -1 in case of y k - y p < 0. Accordingly, what the value β means in view of equation can be regarded as a role of adding an offset to a threshold value used for determination (this point will be described later on with reference to Tables 1 and 2). When the upward divergence (→↑) or the downward divergence (→↓) of the survivor path pattern occurs, there can be confirmed a path to the point (location k) from the point (location p), where a divergence previous to one at the point (location k) thereof, occurs. Repetition of this confirmation allows data to be decoded. FIG. 12 is a block diagram showing the decoder 116 for decoding the data on the basis of the above-described Viterbi algorithm. Reproduced data supplied from the recording channel circuit 112 (shown in FIG. 7) are input to a processing circuit 120 or 130, which processes independently an odd string sample or an even string sample thereof. Then, the respective samples are rearranged in their original orders by a synthesizing circuit 141 on the basis of a timing of a change-over signal output by the change-over circuit 1 and then output. FIG. 12 shows in detail arrangement of the processing circuit 120 for processing the even string sample, while the processing circuit 130 for processing the odd string sample is similarly arranged. In the processing circuit 120, the reproduced data supplied from the recording channel circuit 112 are supplied to a subtracting circuit 11 and a register 12b through a switch 14 which is turned on/off at a timing of the odd string sample / the even string sample in response to the change-over signal output by the change-over circuit 1. In other words, the even string sample of the reproduced data are supplied to the subtracting circuit 11 and the register 12b. The register 12b stores the sample value y p at one previous divergence point. The subtracting circuit 11 subtracts the value y p stored in the register 12b from an input even string sample y k (calculates (y k - y p )) and outputs a result of the subtraction to a comparing circuit 13. The comparing circuit 13 carries out arithmetic processings shown in Tables 1 and 2 in response to the threshold values +2, 0 and -2, the output of the subtracting circuit 11 (y k - y p ) and the value β stored in the register 12a, outputting output data shown in Tables 1 and 2 in response to a result of the calculation. This calculation will be described in detail later on with reference to FIGS. 14 and 15. As shown in FIG. 13, a shift register 121 is formed as a parallel load/serial shift register arranged such that there are connected in parallel to each other a shift register, in which N selectors Sp 1 to Sp N and N flip-flops Dp 1 to Dp N are alternately connected in cascade and a flip-flop Dp 0 is connected at a preceding stage of the first stage selector Sp 1 , and a serial shift register in which N selectors Sm 1 to Sm N and N flip-flops Dm 1 to Dm N are alternately connected in cascade. Reference letter N represents a processing unit length (bit number) used for subjecting the reproduced data (the even string sample) to the Viterbi decoding. A value 0 is input as a signal B or D to the first stage selector Sp 1 or Sm 1 , and a survivor path pattern signal (merge) supplied from the comparing circuit 13 is input as a signal A or C thereto through the flip-flop Dp 0 . Either of the signals (either of the signals A and B or either of the signals C or D) is selected in correspondence with the survivor path pattern signal (merge) and the data (data) supplied from the comparing circuit 13 and then output to the flip-flop Dp 1 or Dm 1 . The survivor path pattern signal (merge) is a flag indicating whether or not a result of discrimination of input condition is the parallel path, in other words, whether or not the state change can be changed. The data (data) is also a flag indicating whether or not the state has already been changed. For example, merge=1 and data=0 indicate that the state could be changed ( or ) but the state has not been changed actually. As shown in Tables 1 and 2, the comparing circuit 13 sets merge = 1 when the upward divergence or the downward divergence occurs and sets merge = 0 when the parallel path occurs. Data latched by the preceding stage flip-flop Dp n-1 are input as the signal A or C to the selectors Sp n or Sm n (n=2, ···, N ) excluding the first stage selector Sp 1 or Sm 1 , and data latched by the preceding stage flip-flop Dm n-1 are input as the signal B or D thereto. Either of the signals A and B or either of the signals C and D is selected in correspondence with the survivor path pattern signal (merge) and the data (data) supplied from the comparing circuit 13 and then output to the succeeding stage flip-flop Dp n+1 or Dm n+1 , respectively. output of selector Sp n output of selector Sm n merge = 0 A D merge = 1 and data = 1 A C merge = 1 and data = 0 B D As shown in Table 3, the selector Sp n (Sm n ) selects and outputs either of the input signals A and B (C and D) in response to the survivor path pattern signal (merge) and the data (data) supplied from the comparing circuit 13. The flip-flop Dp n or Dm n latches an output supplied from the preceding stage selector Sp n or Sm n in synchronism with a PLL clock output from a PLL (not shown). If the arrangement as shown in FIG. 12 is employed, then a squaring circuit becomes unnecessary and it is sufficient to provide one adder and two comparators. Subsequently, operation carried out by the circuit shown in FIG. 12 when a certain signal is input thereto will be explained with reference to FIGS. 14 and 15 which are timing charts. When a signal as shown in FIG. 14 is input to the decoder 116 shown in FIG. 12, the comparing circuit 13 and the shift register 121 (shown in FIG. 13) are respectively operated in accordance with Tables 1 and 2 and Table 3 as follows. Initial values of the values y p and β, however, are set as y p = -2 and β = -1, respectively. <in case of k=0: input y k =y 0 =1.6; y p =-2; β=-1> Since y k - y p = 1.6 - (-2) = 3.6 > 2 is established, an input to the decoder corresponds to the condition pattern F in Table 2. That is, since the upward divergence (sometimes referred to as divergence) occurs, in accordance with Table 2, the value β stored in the register 12a is updated with +1 and the value y p (the sample value at a time when one previous divergence occurred) stored in the register 12b is updated as y p = y 0 = 1.6. At the same time, in accordance with Table 2, the comparing circuit 13 outputs the survivor path pattern signal (merge = 1) and the data (data = 1) to the shift register 121. Accordingly, in the shift register 121 (shown in FIG. 12), the flip-flop Dp 0 latches the signal of merge = 1 (shown in FIG. 15). < in case of k=1: input y k =y 1 =0.2; y p =1.6; β=+1; p=0 > Since -2 ≤ y k - y p = 0.2 -1.6 = -1.4 ≤ 0 is established, the input corresponds to the condition pattern B shown in Table 1. That is, since the parallel path occurs, the values β and y respectively stored in the registers 12a and 12b are set as they are (β = 1 and y p = y 0 ). The comparing circuit 13 outputs the survivor path pattern signal (merge = 0) and the data (data = 0) to the shift register 121. In the shift register 121, the flip-flop Dp 0 latches the signal of merge = 0. Further, since merge = 0 is established, in accordance with Table 3, the signal A of the signal A and B and the signal D of the signals C and D are respectively selected and then output by the selectors Sp n and Sm n to the succeeding stage flip-flops Dp n and Dm n which latch the signals A and D, respectively. In case of the parallel path pattern, a signal (bit) latched by the flip-flop Dp n at an upper stage is latched by the succeeding stage flip-flop Dp n+1 at the same upper stage, and a signal (bit) latched by the flip-flop Dm n at a lower stage is latched by the succeeding stage flip-flop Dm n+1 at the same lower stage. In this case, however, the flip-flop Dm 1 at the lower stage latches a value 0 which is always input to the selector Sm 1 as the signal D. Accordingly, in case of k=1, the upper stage flip-flops Dp 0 and Dp 1 latch values 0 and 1, respectively, and the lower stage flip-flop Dm 1 latches a value 0 (shown in FIG. 15). < in case of k=2: input y k =y 2 =-0.2; y p =1.6; β=+1; p=0 > Since -2 ≤ y k - y p = -0.2 -1.6 = -1.8 ≤ 0 is established, the input corresponds to the condition pattern B shown in Table 1. That is, since the parallel path occurs, the values β and y respectively stored in the registers 12a and 12b are set as they are. The comparing circuit 13 outputs the survivor path pattern signal (merge = 0) and the data (data = 0) to the shift register 121. In the shift register 121, the flip-flop Dp 0 latches the signal of merge = 0. Since merge = 0 is established, in accordance with Table 3, the signal (bit) latched by the flip-flop Dp n at the upper stage is latched by the succeeding stage flip-flop Dp n+1 at the same upper stage, and the signal (bit) latched by the flip-flop Dm n at the lower stage is latched by the succeeding stage flip-flop Dm n+1 at the same lower stage. Accordingly, in case of k=2, the upper stage flip-flops Dp 0 , Dp 1 and Dp 2 latch values 0, 1 and 1, respectively, and the lower stage flip-flops Dm 1 and Dm 2 latch values 0 and 0, respectively (FIG. 15). < in case of k=3: input y k =y 3 =2.0; y p =1.6; β=+1; p=0 > Since y k - y p = 2.0 -1.6 = 0.4 > 0 is established, the input corresponds to the condition pattern C shown in Table 1. That is, since the upward divergence occurs, the previous candidate value y p was defeated by the present value y k (y p < y k was established). In other words, although it was determined that the upward divergence (β = +1) occurred in case of k=0 (p=0), the upward divergence (β = +1) occurred at this time (in case of k=3), so that the parallel path of the upward divergence was selected at the previous time (the path would become discontinuous at k=3 if the upward shift had been selected at k=0). Then, in accordance with Table 1, the vale β stored in the register 12a is set to +1 and the value y p stored in the register 12b is set as y p = y 3 = 2.0. Further, the comparing circuit 13 outputs the survivor path pattern signal (merge = 1) and the data (data = 0) to the shift register 121. In the shift register 121, the flip-flop Dp 0 latches the signal of merge =1. Further, since merge = 1 and data = 0 are established, in accordance with Table 3, the selector Sp n or Sm n respectively selects the signal B or the signal D of the signals A and B or the signals C and D, which is respectively output therefrom to and then latched by the succeeding stage flip-flop Dp n or Dm n . If the just previous divergence was the upward divergence (β = +1 was established) and further the present divergence is the upward divergence, then it is determined that the signal (bit) latched by the upper stage flip-flop Dp n as a decoding data candidate is defeated and hence the signal (bit) latched by the lower stage flip-flop Dm n is latched by the succeeding stage flip-flops Dp n+1 and Dm n+1 at the upper and lower stages. In this case, however, the upper stage flip-flop Dp 1 latches the value 0 which is always input to the selector Sp 1 as the signal B. Accordingly, in case of k=3, the upper stage flip-flops Dp 0 , Dp 1 , Dp 2 and Dp 3 latch the values 1, 0, 0 and 0, respectively, and the lower stage flip-flops Dm 1 , Dm 2 and Dm 3 latch the values 0, 0 and 0, respectively (as shown in FIG. 15). < in case of k=4: input y k =y 4 =0.2; y p =2.0; β=+1; p=3 > Since -2 ≤ y k - y p = 0.2 -2.0 = -1.8 ≤ 0 is established, the input corresponds to the condition pattern B shown in Table 1. That is, since the parallel path occurs, the values β and y p respectively stored in the registers 12a and 12b are set as they are. The comparing circuit 13 outputs the survivor path pattern signal (merge = 0) and the data (data = 0) to the shift register 121. In the shift register 121, the flip-flop Dp 0 latches the signal of merge = 0. Since merge = 0 is established, the signal (bit) latched by the flip-flop Dp n at the upper stage is latched by the succeeding stage flip-flop Dp n+1 at the same upper stage, and the signal (bit) latched by the flip-flop Dm n at the lower stage is latched by the succeeding stage flip-flop Dm n+1 at the same lower stage. < in case of k=5: input y k =y 5 =-0.4; y p =2.0; β=+1; p=3 > Since y k - y p = -0.4 -2.0 = -2.4 < -2 is established, the input corresponds to the condition pattern A shown in Table 1. That is, since the downward divergence occurs, it is determined that the previous candidate value is correct (that is, in case of k=3 (p=3), the upward shift of the upward divergence was selected.). Therefore, in accordance with Table 1, the value β stored in the register 12a is set to -1 and the value y p stored in the register 12b is set as y p = y 5 = -0.4. Further, the comparing circuit 13 outputs the survivor path pattern signal (merge = 1) and the data (data = 1) to the shift register 121. In the shift register 121, since the flip-flop Dp 0 latches the signal of merge =1 and further merge = 1 and data = 1 are established, in accordance with Table 3, the selectors Sp n and Sm n respectively select the signal A of the signals A and B and the signal C of the signals C and D, which are respectively output therefrom to and then latched by the succeeding stage flip-flops Dp n and Dm n . When the previous divergence was the upward divergence (β =+1 was established) and further the present divergence is the downward divergence, it is determined that the signal (bit) latched by the upper stage flip-flop Dp n as the decoding data candidate is correct and hence the signal (bit) latched by the upper stage flip-flop Dp n is latched by the succeeding stage flip-flops Dp n+1 and Dm n+1 at the upper stage and lower stage. < in case of k=6: input y k =y 6 =-0.2; y p =-0.4; β=-1; p=5> Since 0 ≤ y k - y p = -0.2 -(-0.4) = 0.2 ≤ +2 is established, the input corresponds to the condition pattern E shown in Table 2. That is, since the parallel path occurs, the values β and y p are set as they are. The comparing circuit 13 outputs the survivor path pattern signal (merge = 0) and the data (data = 0) to the shift register 121. In the shift register 121, the flip-flop Dp 0 latches the signal of merge = 0. Since merge = 0 is established, the signal (bit) latched by the flip-flop Dp n at the upper stage is latched by the succeeding stage flip-flop Dp n+1 at the same upper stage, and the signal (bit) latched by the flip-flop Dm n at the lower stage is latched by the succeeding stage flip-flop Dm n+1 at the same lower stage. < in case of k=7: input y k =y 7 =-2.0; y p =-0.4; β=-1; p=5> Since y k - y p = -2.0 -(-0.4) = -1.6 < 0 is established, the input corresponds to the condition pattern D shown in Table 2. That is, since the downward divergence occurs, it is determined that the previous candidate value was defeated. That is, in case of k=5 (p=5), not the downward shift but the parallel shift was selected. Therefore, in accordance with Table 2, the value β stored in the register 12a is set to -1 and the value y p stored in the register 12b is set as y p = y 7 = -2.0. Further, the comparing circuit 13 outputs the survivor path pattern signal (merge = 1) and the data (data = 0) to the shift register 121. In the shift register 121, since the flip-flop Dp 0 latches the signal of merge =1 and further merge = 1 and data = 0 are established, in accordance with Table 3, the selectors Sp n and Sm n respectively select the signal B of the signals A and B and the signal D of the signals C and D, which are respectively output therefrom to and then latched by the succeeding stage flip-flops Dp n and Dm n . If the just previous divergence was the downward divergence (β = -1 was established) and further the present divergence is the downward divergence, then it is determined that the signal (bit) latched by the upper stage flip-flop Dp n as the decoding data candidate was defeated and hence the signal (bit) latched by the lower stage flip-flop Dm n is latched by the succeeding stage flip-flops Dp n+1 and Dm n+1 at the upper and lower stages. In this case, however, the upper stage flip-flop Dp 1 latches the value 0 which is always input to the selector Sp 1 as the signal B. <in case of k=8: input y k =y 8 = 0.2; y p = -2.0; β=-1; p=7> Since y k - y p = 0.2 -(-2.0) = 2.2 > +2 is established, the input corresponds to the condition pattern F shown in Table 2. That is, since the upward divergence occurs, it is determined that the previous data is correct. In other words, in case of k=7 (p=7), the downward shift occurred. Therefore, in accordance with Table 2, the value β stored in the register 12a is set to 1 and the value y p stored in the register 12b is set as y p = y 8 = 0.2. Further, the comparing circuit 13 outputs the survivor path pattern signal (merge = 1) and the data (data = 1) to the shift register 121. In the shift register 121, the flip-flop Dp 0 latches the signal of merge =1. Further, since merge = 1 and data = 1 are established, in accordance with Table 3, the selector Sp n or Sm n respectively selects the signal A or the signal C of the signals A and B or the signals C and D, which is respectively output therefrom to and then latched by the succeeding stage flip-flop Dp n or Dm n . When the previous divergence was the downward divergence (β = -1 was established) and further the present divergence is the upward divergence, it is determined that the signal (bit) latched by the upper stage flip-flop Dp n as the decoding data candidate is correct and hence the signal (bit) latched by the upper stage flip-flop Dp n is latched by the succeeding stage flip-flops Dp n+1 and Dm n+1 at the upper stage and lower stage. Hereinafter, the data are decoded similarly. A bit for causing the condition A or C shown in Table 1 or the condition D or F shown in Table 2 is added to the end of a bit string. If the condition A or C shown in Table 1 or the condition D or F shown in Table 2 is caused, then the values stored by the upper stage flip-flops Dp 1 to Dp N and the lower stage flip-flops Dm 1 to Dm N are respectively matched with each other. Therefore, either of the data (bit) latched by the upper stage flip-flop Dp N and the lower flip-flop Dm N (for example, by the upper stage flip-flop Dp N ) are successively received to thereby obtain the Viterbi-decoded data. By the way, a magnetic recording and reproducing apparatus or an optical recording and reproducing apparatus, for example, adds an error detecting code such as a CRC (Cyclic Redundancy Check) code, for example, or the like to an ID such as a sector number, a track number or the like in order to improve reliability. If an equation G(x) = x 16 + x 12 + x 5 + 1 is used as a generating polynomial G(x) of the CRC code, for example, then data of a predetermined bit length BL are divided by the generating polynomial G(x) = x 16 + x 12 + x 5 +1 and a remainder of the division is added to an end of the data. When an error of the data added with such CRC code is detected, a CRC decoding circuit as shown in FIG. 16 is used. In the CRC decoding circuit, there are connected in series flip-flops D 1 to D 16 of the same number as the highest degree of the generating polynomial (which is 16 in case of the generating polynomial shown in equation (6)) and there are provided XOR gates (which are indicated by a symbol composed by inserting a symbol + into a symbol ○ in the figure) at the preceding stage of the flip-flop D 1 , between the flip-flops D 5 and D 6 and between the flip-flops D 12 and D 13 . Further, to the XOR gate provided at the preceding stage of the flip-flop D 1 , the decoded data are input and further an output from the flip-flop D 16 is fed back. An output of the above XOR gate is input to not only the flip-flop D 1 but also the XOR gates provided between the flip-flops D 5 and D 6 and between the flip-flops D 12 and D 13 . In the CRC decoding circuit thus arranged, an exclusive-OR of a decoded bit string of the predetermined bit length (the bit length of data including the CRC code) BL and the output from the flip-flop D 16 is calculated at the XOR gate provided at the preceding stage of the flip-flop D 1 . While the exclusive-OR of the decoded bit string and the output of the XOR gate provided at the preceding stage of the flip-flop D 1 is being calculated at the XOR gates provided between the flip-flops D 5 and D 6 and between the flip-flops D 12 and D 13 , the decoded bit string is successively latched by the flip-flops D 1 to D 16 , whereby when the last bit of the bit string is input to the flip-flop D 1 , the bit string is subjected to a calculation (CRC calculation) in which the bit string is divided by the generating polynomial shown in equation (6). As a result, if the bits latched by the flip-flops D 1 to D 16 all have values 0, i.e., the decoded bit string of the predetermined bit length BL is completely divided by the generating polynomial shown in equation (6), then it is determined that the bit string has no error. If any one of the bits latched by the flip-flops D 1 to D 16 is not a value 0, i.e., the decoded bit string of the predetermined bit length BL is not completely divided by the generating polynomial shown in equation (6), then it is determined that the bit string has an error. By the way, when the data are decoded by the Viterbi decoder 116 arranged as shown in FIG. 12, it is necessary that the data (bit) are successively shifted by the shift register 21 shown in FIG. 13. Therefore, a processing time is delayed to an extent of the bit number of the number of the flip-flop Dp n (Dm n ) provided in series in the shift register 121. When the error of the decoded data is detected by the CRC decoding circuit as shown in FIG. 16, it is necessary that the flip-flops D 1 to D 16 successively latch the bit strings of the predetermined bit length BL. Therefore, a processing time is delayed to an extent of the predetermined bit length BL. Accordingly, when the error of the data decoded by the Viterbi decoder 116 is detected by the CRC decoding circuit as shown in FIG. 16, the processing time is considerably delayed. The Viterbi decoder 116 and the CRC decoding circuit are not useful for a portion where there is required quick judgement whether or not data are read from or written on a sector after an ID is decoded, such as an ID portion of the sector or the like. However, if they are applied to such portion in order to improve reliability of the apparatus, then, as shown in FIG. 17, a long gap must be provided between an end of an ID region of the recording medium corresponding to a timing when the input of the reproduced data of the ID to the Viterbi decoder 116 is terminated and a beginning of a data region corresponding to a timing when a CRC operation in the CRC decoding circuit is terminated (input of the decoded data of the ID from the Viterbi decoder 116 to the CRC decoding circuit is terminated). Therefore, there is then the problem that a recording capacity of the recording medium is lowered. In "Viterbi Detection of Class IV Partial Response on a Magnetic Recording Channel", R.W. Woed and D.A. Petersen, IEEE Transaction on Communications, vol. com-34, no.5, May 1986, XP577357, in more detail is descriped such partial response as discussed above. However nothing is disclosed what to do if the encoded data additionally comprise an error detection code. The disclosed Viterbi-decoder uses a RAM as an intermediate memory through which pass the Viterbi-decoded data. US-A-4 680 764 only discloses that besides CIRC-decoding a CRC-check may be carried out, both using data stored in a RAM. In "Error-Correcting Codes", second edition, by W.W Peterson and E.J. Weldon, MIT Press, 1978, pp. 224-230, is disclosed that error detection with cyclic codes, like a CRC-code, may be based on an encoding using a multe stage shift register and binary-adding received check digits. It is an object of the present invention to minimize operation time when combining maximum likelihood decoding and error detection by CRC-decoding and to enlarge the storing capacity of the recording medium. The solution of this object is set forth in claim 1. Further features are set forth in the dependent claims. The basic idea is to use the same shift register means comprising flip-flops as latch means and exclusive-OR-gates (XOR) as arithmetic means, when performing both maximum likelihood decoding and error detection. According to the information reproducing apparatus arranged as described above, the XOR gates for calculating the exclusive-ORs of the outputs of the flip-flops are desposed between the adjacent flip-flops connected in series to each other for forming the shift register means for effecting a Viterbi decoding method to carry out the CRC calculation. Accordingly, since the Viterbi decoding and the CRC operation are carried out simultaneously, a delay time necessarily caused by the data decoding and the error correction can be drastically reduced, with the result that the recording capacity of the recording medium can be improved. BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a block diagram showing construction of an information reproducing apparatus according to one embodiment of the present invention. FIG. 2 is a block diagram showing in detail a shift register arithmetic circuit 2 shown in FIG. 1. FIG. 3 is a diagram showing concrete arrangement of signal generating circuits (30a to 30d). FIG. 4 is a timing chart used to explain operation of the embodiment shown in FIG. 1. FIG. 5 is a timing chart used to explain a Viterbi decoding and a CRC operation in the embodiment shown in FIG. 1. FIG. 6 is a block diagram used to explain a partial response modulation. FIG. 7 is a block diagram showing construction of a recording and reproducing system of the partial response. FIG. 8 is a graph showing change of a signal level. FIG. 9 a state transition diagram of a partial response PRS (1, -1). FIG. 10 is a trellis diagram of the state transition diagram shown in FIG. 9. FIG. 11 is a diagram used to explain a Viterbi algorithm. FIG. 12 is a block diagram showing construction of one example of a decoder 116 using the Viterbi algorithm. FIG. 13 is a block diagram showing in detail a shift register 121 of the decoder 116 shown in FIG. 12. FIG. 14 is a timing chart used to explain operation of the decoder 116 shown in FIG. 12. FIG. 15 is a timing chart used to explain operation of the shift register 121 shown in FIG. 13. FIG. 16 is a block diagram showing construction of one example of a CRC decoding circuit for carrying out a CRC operation. FIG. 17 is a timing chart used to explain timings of the Viterbi decoding and the CRC operation in a conventional apparatus. BEST MODE CARRYING OUT THE INVENTION FIG. 1 is a block diagram showing construction of the information reproducing apparatus according to one embodiment of the present invention. In the figure, the same reference numerals are given to portions corresponding those shown in FIG. 12. In the apparatus, a processing circuit 10 for processing an even string sample has an arrangement in which the shift register 121 is removed from a processing circuit 120 shown in FIG. 12, and a shift register arithmetic circuit 2 is provided instead of the synthesizing circuit 141. A processing circuit 20 for processing an odd string sample is formed of a subtracting circuit 21, registers 22a and 22b, a comparing circuit 23 and a switch 24 which are respectively formed similarly to a subtracting circuit 11, registers 12a and 12b, a comparing circuit 13 and a switch 14 of the processing circuit 10. Further, as shown in FIG. 1, a value β, a signal merge and data data output from the comparing circuit 13 of the processing circuit 10 for processing the even string sample are added at their ends of letters with reference letters _even in order to indicate that they are signals corresponding to the even string sample. A value β, a signal merge and data data output from the comparing circuit 23 of the processing circuit 20 for processing the odd string sample are added at their ends of letters with reference letters_ odd in order to indicate that they are signals corresponding to the odd string sample. The shift register arithmetic circuit 2 is arranged as shown in FIG. 2 and subjects reproduced data to a Viterbi decoding on the basis of an algorithm using the above-mentioned differential metric and to a CRC operation simultaneously. If the highest degree of a generating polynomial in the CRC operation is set to j , then the shift register arithmetic circuit 2, as shown in FIG. 2, is formed as a parallel load / serial shift register in which there are connected in parallel serial shift registers of four series a to d respectively arranged such that j + 1 selectors Sa 0 to Sa j , Sb 0 to Sb j , Sc 0 to Sc j and Sd 0 to Sd j are connected between j + 2 flip-flops Da -1 to Da j , Db -1 to Db j , Dc -1 to Dc j and Dd -1 to Dd j connected in series. The flip-flops Da -1 to Da j , Db -1 to Db j , Dc -1 to Dc j and Dd -1 to Dd j latch input data at a timing in which a clock (shown in FIG. 4(a)) is supplied. The selectors Sa 0 to Sa j , Sb 0 to Sb j , Sc 0 to Sc j and Sd 0 to Sd j select and output one of three input signals respectively input on the basis of a change-over signal supplied from a change-over circuit 1 (even/odd -1 (which is represented in FIGS. 1 to 4 by adding a bar (-) to a portion above odd), merge_even and data_even supplied from the comparing circuit 10 and merge_odd and data_odd supplied from the comparing circuit 20. In this embodiment, the generating polynomial in the CRC operation is expressed by G(x) shown in the above equation (6). Accordingly, the value j is set to 16. Further, the shift register arithmetic circuit 2 is arranged such that XOR gates 31a to 31d are respectively provided between the flip-flops Da 0 , Db 0 , Dc 0 and Dd 0 and the selectors Sa 1 , Sb 1 , Sc 1 and Sd 1 , XOR gates 32a to 32d are respectively provided between the flip-flops Da 5 , Db 5 , Dc 5 and Dd 5 and the selectors Sa 6 , Sb 6 , Sc 6 and Sd 6 , XOR gates (not shown) are respectively provided between the flip-flops Da 12 , Db 12 , Dc 12 and Dd 12 and the selectors Sa 13 , Sb 13 , Sc 13 and Sd 13 , and outputs of the flip-flops Da 16 , Db 16 , Dc 16 and Dd 16 are respectively input (fed back) to the XOR gates 31a to 31d. The shift register arithmetic circuit 2 is also arranged such that outputs of the XOR gates 31a to 31d are respectively input to the XOR gates 32a to 32d and to the XOR gates provided between the flip-flops Da 12 , Db 12 , Dc 12 and Dd 12 and the selectors Sa 13 , Sb 13 , Sc 13 and Sd 13 . Accordingly, each of the serial shift registers of the four series a to d in the shift register arithmetic circuit 2 is arranged similarly to one provided with the selectors at the preceding stages of the respective flip-flops of the CRC decoding circuit corresponding to the generating polynomial of equation (6) of FIG. 16 and with two flip-flops and one selector at the preceding stage of the XOR gate provided at the preceding stage of the flip-flop D 1 . In other words, if attention is given to each of the serial shift registers of the four series a to d of the shift register arithmetic circuit 2, then each of the serial shift registers carries out the CRC operation based on the generating polynomial G(x) shown in equation (6). If a circuit in which all the XOR gates are removed from the shift register arithmetic circuit 2 is supposed, then the circuit is one which is developed from the shift register 121 shown in FIG. 13 for processing the even string sample and simultaneously processes the even string sample and the odd string sample. In other words, the circuit which is formed by removing all the XOR gates from the shift register arithmetic circuit 2, Viterbi-decodes reproduced data successively (in order of samples) and outputs them. Subsequently, operation thereof will be explained. A pair of circuits (for example, arithmetic circuits 102 and 103 shown in FIG. 6 (b)) for calculating a partial response (1,-1), for example, are used with being interleaved to thereby supply the reproduced data to the processing circuits 10 and 20. On the other hand, the change-over circuit 1 supplies a selection signal (even/odd -1 ) (shown in FIG. 4(b)) alternately changed to an H level (logic 1) / an L level (logic 0) in accordance with a timing of a leading edge of the clock (shown in FIG. 4(a)) as shown in FIG. 4 to the shift register arithmetic circuit 2 and the processing circuits 10 and 20. The processing circuits 10 and 20 respectively process the reproduced data of the even sample string and the odd sample string input through switches 14 and 24 at the timing of the selection signal (even/odd -1 ) supplied from the change-over circuit 1 similarly to processing explained with reference to FIG. 12, respectively outputting the signals merge_even and data_even and the signals merge_odd and data_odd from the comparing circuits 13 and 23 to the shift register arithmetic circuit 2 in accordance with the above-mentioned Tables 1 and 2. When the selection signal (even/odd -1 ) supplied from the change-over circuit 1 indicates the logic 1 (when it is at H level), the shift register arithmetic circuit 2 carries out processing on the basis of the signals merge_even and data_even output from the comparing circuit 13 at the timing shown in FIG. 4(c). When the selection signal (even/odd -1 ) supplied from the change-over circuit 1 indicates the logic 0 (when it is at L level), the shift register arithmetic circuit 2 carries out processing on the basis of the signals merge_odd and data_odd output from the comparing circuit 23 at the timing shown in FIG. 4(d). The shift register arithmetic circuit 2 generates in signal generating circuits (30a to 30d) the four signals (input_a, input_b, input_c and input_d) shown in the following equations from the selection signal (even/odd - ) supplied from the change-over circuit 1, the signals merge_even and data_even output from the comparing circuit 13 and the signals merge_odd and data_odd output from the comparing circuit 23. To speak concretely, the circuits 30a to 30d are the circuits shown in FIGS. 3(a) to 3(d). input_a=(even/odd -1 =1)*merge_even+(even/odd -1 =0)*merge_odd input_b=(even/odd -1 =1)*merge_even input_c=(even/odd -1 =0)*merge_odd input_d=0 where symbols * and + represent conjunction and disjunction, respectively, further (even/odd -1 =1) has a logic 1 when the signal even/odd -1 is a logic 1 (in case of the timing of the even string sample) and has a logic 0 when the signal even/odd -1 is a logic 0 (in case of the timing of the odd string sample), and (even/odd -1 =0) has a logic 0 when the signal even/odd -1 is a logic 1 and has a logic 1 when the signal even/odd -1 is a logic 0. Accordingly, the signal input_b has a value which is valid only at the timing of the even string sample and the same as the signal merge (merge_even) output from the comparing circuit 13, and the signal input_c has a value which is valid only at the timing of the odd string sample and the same as the signal merge (merge_odd) output from the comparing circuit 23. Further, the signal input_a has the same value as the signal merge (merge_even) output from the comparing circuit 13 at the timing of the even string sample and has the same value as the signal merge (merge_odd) output from the comparing circuit 23 at the timing of the odd string sample. The signal input_d always has a value 0. Summarization of the above equations is as follows. (1) as to the signal input_a 1 ○ if even/odd -1 (hereinafter referred to as e/ o )= 1 and merge_e = 1, then input_a =1. 2 ○ if e/ o = 1 and merge_e = 0, then input_a =0. 3 ○ if e/ o = 0 and merge_o = 1, then input_a =1. 4 ○ if e/ o = 0 and merge_e = 0, then input_a =0. (2) as to the signal input_b 1 ○ if e/ o = 1 and merge_e = 1, then input_b =1. 2 ○ if e/ o = 1 and merge_e = 0, then input_b =0. 3 ○ if e/ o = 0, then input_b =0. (3) as to the signal input_c 1 ○ if e/ o = 0 and merge_o = 1, then input_c =1. 2 ○ if e/ o = 0 and merge_o = 0, then input_c =0. 3 ○ if e/ o = 1, then input_c =0. (4) as to the signal input_d input_d = 0 is always established. The four signals input_a, input_b, input c and input_d are respectively input to the flip-flops Da -1 to Dd -1 at the first stage of the shift register arithmetic circuit 2 (shown in FIG. 2). The respective signals input_a, input_b, input_c and input_d respectively input to the flip-flops Da -1 to Dd -1 are sequentially latched by the succeeding stage flip-flops through the selectors. If the signals supplied from the preceding stages of the shift registers of the series a to d are respectively expressed as in_a, in_b, in_c and in_d, then the selectors Sa j , Sb j , Sc j and Sd j ( j = 0, 1, ···, j (which is set as j = 16 as described above in this embodiment)) output signals out_a, out_b, out_c and out_d in accordance with the following equations, respectively. out_a= (even/odd -1 =1)*(merge_even=1)*(data_even=0)*in_c +(even/odd -1 =1)*((merge_even=1)*(data_even=0)) -1 *in_a +(even/odd -1 =0)*(merge_odd=1)*(data_odd=0)*in_b +(even/odd -1 =0)*((merge_odd=1)*(data_odd=0)) -1 *in_a out_b= (even/odd -1 =1)*(merge_even=1)*(data_even=0)*in_d +(even/odd -1 =1)*((merge_even=1)*(data_even=0)) -1 *in_b +(even/odd -1 =0)*(merge_odd=1)*(data_odd=1) -1 *in_a +(even/odd -1 =0)*((merge_odd=1)*(data_odd=1)) -1 *in_b out_c= (even/odd -1 =1)*(merge_even=1)*(data_even=1)*in_a +(even/odd -1 =1)*(merge_even=1)*(data_even=1) -1 *in_c +(even/odd -1 =0)*(merge_odd=1)*(data_odd=0)*in_d +(even/odd -1 =0)*((merge_odd=1)*(data_odd=0)) -1 *in_c out_d= (even/odd -1 =1)*(merge_even=1)*(data_even=1)*in_b +(even/odd -1 =1)*((merge_even=1)*(data_even=1)) -1 *in_d +(even/odd -1 =0)*(merge_odd=1)*(data_odd=1)*in_c +(even/odd -1 =0)*((merge_odd=1)*(data_odd=1)) -1 *in_d Symbols () -1 represent a denial of equation in parenthesis. That is, the symbols () -1 has a value 0 when the logic in the parenthesis is 1 and has a value 1 when the logic in the parenthesis is 0. Summarization of the above equations is as follows. (1) as to the signal out_a 1 ○ the timing of the even sample (a) if merge_e = 1 and data_e = 0, then out_a = in_c. (b) if merge_e = 0 or data_e = 1, then out_a = in_a. 2 ○ the timing of the odd sample (a) if merge_o = 1 and data_o = 0, then out_a = in_b. (b) if merge_o = 0 or data_o = 1, then out_a = in_a. (2) as to the signal out_b 1 ○ the timing of the even sample (a) if merge_e = 1 and data_e = 0, then out_b = in_d. (b) if merge_e = 0 or data_e = 1, then out_b = in_b. 2 ○ the timing of the odd sample (a) if merge_o = 1 and data_o = 1, then out_b = in_a. (b) if merge_o = 0 or data_o = 0, then out_b = in_b. (3) as to the signal out_c 1 ○ the timing of the even sample (a) if merge_e = 1 and data_e = 1, then out_c = in_a. (b) if merge_e = 0 or data_e = 0, then out_c = in_c. 2 ○ the timing of the odd sample (a) if merge_o = 1 and data_o = 0, then out_c = in_d. (b) if merge_o = 0 or data_o = 1, then out_c = in_c. (4) as to the signal out_d 1 ○ the timing of the even sample (a) if merge_e = 1 and data_e = 1, then out_d = in_b. (b) if merge_e = 0 or data_e = 0, then out_d = in_d. 2 ○ the timing of the odd sample (a) if merge_o = 1 and data_o = 1, then out_d = in_c. (b) if merge_o = 0 or data_o = 0, then out_d = in_d. Similarly to the case explained with reference to FIG. 13, on the basis of the above equations, the shift register arithmetic circuit 2 selects a series of the serial shift register to survive (a correct path) in response to the signals merge_even and data_even supplied from the comparing circuit 10 and the signals merge_odd and data_odd supplied from the comparing circuit 20, and the data latched by the flip-flops of the selected series of the serial shift register are copied to the flip-flops of other series of the serial shift register to thereby effect the error correction based on the Viterbi decoding method. Simultaneously, in the shift register arithmetic circuit 2, the XOR gates 31a to 31d calculate exclusive-ORs of outputs of the flip-flops Da 16 to Dd 16 at the last stages of the serial shift registers of the series a to d and outputs of the flip-flops Da 0 to Dd 0 thereof and then input the results to the selectors Sa 1 to Sd 1 , respectively. Further, exclusive-ORs of the outputs of the XOR gates 31a to 31d and outputs of the flip-flops Da 5 to Dd 5 of the serial shift registers of the series a to d are calculated by the XOR gates 32a to 32d and then input to the selectors Sa 6 to Sd 6 , respectively. Exclusive-ORs of the outputs of the former XOR gates 31a to 31d and outputs of the flip-flops Da 12 to Dd 12 , not shown, of the serial shift registers of the series a to d are calculated by the XOR gates respectively provided between the flip-flops Da 12 to Dd 12 and the selectors Sa 13 to Sd 13 , not shown, and then input to the selectors Sa 13 to Sd 13 , respectively. Accordingly, the shift register arithmetic circuit 2 carries out the CRC operation based on the generating polynomial shown in equation (6) similarly to the case explained with reference to FIG. 16. By the way, when the partial response (1,0,-1) is Viterbi-decoded, it is necessary to add a two-bit code for terminating a trellis to an end of a block (a bit string of a processing unit to be decoded) of the data (bit string) to be decoded. The two-bit code is generally added to the end of the block as code (1,1) in the form of the code before pre-coding. The two-bit code for terminating the trellis is not necessary for the CRC operation. Accordingly, when two bit data which are added to the end of data block and correspond to the code (1,1) for terminating the trellis are respectively latched by the flip-flops Da -1 to Dd -1 and Da 0 to Dd 0 of the serial shift registers of the series a to d , the shift register arithmetic circuit 2 estimates the result of the CRC operation on the basis of the exclusive-ORs of 16 bits latched by any series of the flip-flops Da 1 to Da 16 , Db 1 to Db 16 , Dc 1 to Dc 16 and Dd 1 to Dd 16 . When all the 16 bits latched by any series of the flip-flops Da 1 to Da 16 , Db 1 to Db 16 , Dc 1 to Dc 16 and Dd 1 to Dd 16 have values 0, the result of the CRC operation is regarded as one indicating that data have no error in the determining circuit 34 shown in FIG. 12, while when any of 16 bits does not have the value 0, the result of the CRC operation is regarded as one indicating that the data have error. As described above, in order to carry out the CRC operation, the XOR gates for calculating the exclusive-ORs of the outputs of the flip-flops of the different series are disposed between the flip-flops connected in series which form the respective series of the serial shift registers for effecting the Viterbi decoding method. Therefore, when the generating polynomial of j order of the CRC code is used, the result of the CRC operation can be obtained within j -1 clocks after the last bit of the reproduced data block is input to the processing circuits 10 and 20 shown in FIG. 1. As shown in FIG. 5, the Viterbi decoding and the CRC operation are simultaneously performed, so that the delay time inevitably caused by the data decoding and the error correction can be drastically reduced and the gap provided between the ID portion and the data portion of the recording medium can be made smaller. Accordingly, the data can be recorded on the recording medium with high density, and the highly reliable data can be reproduced from the recording medium recorded with high density. The present invention can be applied to apparatus for recording and reproducing information on and from the magnetic recording medium such as the magnetic tape apparatus, the magnetic disc apparatus or the like, for example, as long as the apparatus employ the partial response system and employs the CRC code as the error correction code. Further, the present invention can be applied to apparatus for recording and reproducing information on and from an optical recording medium such as a magneto-optical disc apparatus, an optical disc apparatus, an optical card apparatus or the like. While the generating polynomial of the CRC shown in equation (6) is used in this embodiment, equation capable of being used in the present invention is not limited to this equation and other equations can be used in the present invention. In this case, the shift register arithmetic circuit 2 may be arranged such that the number of the stages of the flip-flops thereof is increased or decreased and the number and insertion positions of the XORs are changed in response to a generating polynomial to be used.

Claims: An information reproducing apparatus which obtains reproduced data from a recording medium by utilizing a partial response system and decodes said reproduced data on the basis of a maximum likelihood decoding method by using a plurality of latch means (D i ; i= -1,0,..n), said reproduced data further comprising an error detecting code (CRC), said apparatus having means for a calculation in accordance with a generating polynomicial (G(X)) to decode said error detection code, said means for calculation comprising said latch means connected to each other in series, and arithmetic means (S j ; j= 0,1, n) comprising exclusive OR means (31, 32) and being disposed between adjacent ones of said latch means, said latch means (D i ) and said arithmetic means (S j ) forming a shift register means (2). Information reproducing apparatus according to claim 1 wherein a plurality of serial shift register means (2), each comprising said latch means (D ai , D bi ,..) and said arithmetic means (S aj , S bj ,..) are connected in parallel with the same number of stages (i, j,). Information reproducing apparatus according to claim 1 or 2, wherein, if the maximum degree of said generating polynominal (G(X)) in said error detection calculation is j, then said shift register means (D i , S j ) includes said latch means in at least j+2 stages and a result of said error detection calculation is estimated on the basis of values, whose number is smaller than j+2, of values latched by said latch means of at least j+2 stages. Information reproducing apparatus according to claim 3 wherein said shift register means (2; D i S j ) estimates a result of said error detection calculation on the basis of values latched by the latter j stage latch means of values latched by said latch means of at least j+2 stages. Information reproducing apparatus according to anyone of claims 1 to 4, wherein said data is recorded on said recording medium by using a partial response system of class IV, and wherein said shift register means (2; D i S j ) subjects said data to a maximum likelihood decoding and carries out said error detection calculation by using and interleaving processed results of a pair of processing means (10, 20) for performing processing with a partial response (1, -1). Information reproducing apparatus according to anyone of claims 1 to 5, further comprising change-over means (14,24) for generating a change-over signal for switching samples of said reproduced data from an odd string sample to an even string sample and vice versa, wherein said shift register means (2) simultaneously processes the even string sample and the odd string sample based on a processing result of said processing means (10, 20) on the basis of said change-over signal. Information reproducing apparatus according to anyone of claims 1 to 6, wherein said maximum likelihood decoding is a Viterbi decoding. Information reproducing apparatus according to anyone of claims 1 to 7, wherein said error detection code is a CRC-code. Information reproducing apparatus claimed in anyone of claims 3 to 7, wherein an error is detected by judging whether or not all values latched by the latter j stage latch means of j+2 values latched by said j +2 stage latch means are the same. Information reproducing apparatus according to anyone of claims 5 to 9, wherein said processing means (10, 20) calculates a differential metric from reproduced data and, on the basis of said differential metric, outputs as processing results a first flag indicating probability of state change and a second flag indicating that the state has actually been changed.

Inventors: HAYASHI NOBUHIRO, HAYASHI, NOBUHIRO, HAYASHI, NOBUHIRO SONY CORPORATION
Applicants: SONY CORP, SONY CORPORATION
Application year: 1994
Main classification: G11B20
IPC codes: H03M13, G11B20